Gas turbine engines, such as those used to power modern commercial aircraft or in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor.
An exemplary combustor features an annular combustion chamber defined between a radially inboard liner and a radially outboard liner extending aft from a forward bulkhead. The radially outboard liner extends circumferentially about and is radially spaced from the inboard liner, with the combustion chamber extending fore to aft therebetween. Exemplary liners are double structured, having an inner heat shield and an outer shell. A plurality of circumferentially distributed fuel injectors and associated swirlers or air passages is mounted in the forward bulkhead. The fuel injectors project into the forward end of the annular combustion chamber to supply the fuel to be combusted. The swirlers impart a swirl to primary combustion air entering the forward end of the combustion chamber at the bulkhead to provide rapid mixing of the fuel and combustion air. One or more rows of circumferentially distributed combustion air holes that penetrate the outboard liner and the inboard liner at one or more axial locations may be provided to admit additional air into the combustion chamber along the length of the combustion chamber. Commonly assigned U.S. Pat. Nos. 7,093,441; 6,606,861 and 6,810,673, the entire disclosures of which are hereby incorporated herein by reference as if set forth herein, disclose exemplary prior art annular combustors for gas turbine engines.
Combustion of the hydrocarbon fuel in air inevitably produces oxides of nitrogen (NOx). NOx emissions are the subject of increasingly stringent controls by regulatory authorities. Accordingly, engine manufacturers strive to minimize NOx emissions. One combustion strategy for minimizing NOx emissions from gas turbine engines is commonly referred to as lean direct injection (LDI) combustion. The LDI combustion strategy recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e. when the fuel-air ratio is at or near stoichiometric.
In LDI combustion, more than the stoichiometric amount of air required for combustion of the fuel is injected into the forward region of the combustion chamber and rapidly mixed with the fuel to combust via a fuel-lean, as opposed to fuel-rich, process. A combustor configured for LDI combustion includes two serially arranged combustion zones: an overall fuel-lean combustion zone at the forward end of the combustor in which fuel and air are initially mixed followed by additional lean-burn combustion and mixing supported by dilution or cooling air addition in the axially aft portion of the combustor. The combustion process in a combustor configured for LDI combustion, by design intent, exists in one bulk governing state in which combustion is exclusively stoichiometric fuel lean. Clearly, local conditions may not be lean given that mixing of the fuel and air require some finite time and spatial volume via mixing to achieve this state. However, overall combustion occurs under fuel lean conditions, that is at an equivalence ratio less than 1.0. The substantial excess of air in the forward combustion zone inhibits NOx formation by suppressing the combustion flame temperature.
In gas turbine operations, the overall combustion fuel air ratio is determined by the power demand on the engine. At low power demand, the combustor is fired at a relatively low fuel air ratio. At high power demand, the combustor is fired at a relatively high fuel air ratio. Adjustment of the fuel air ratio in response to power demand is achieved by varying the amount of fuel delivered through the fuel injectors and injected into the forward region of the combustion chamber while maintaining the amount of airflow admitted into the forward region of the combustion chamber. The capability of operating gas turbine engines having conventional combustors with LDI combustion has proved to be somewhat limited at low fuel air ratios due to reduced combustion efficiency and fuel lean combustion stability concerns.